1. Field of The Invention
The present invention relates to a control appratus for an automatic transmission in automotive vehicles, and more particularly to a, control apparatus for an automatic transmission which includes a plurality of frictional engagement elements arranged to be selectively engaged or disengaged for effecting a change in speed of the transmission.
2. Description of the Prior Art
In Japanese Patent Laid-open Publication No. 3-117766, there has been proposed a control apparatus for an automatic transmission of the type which includes a set of frictional engagement elements arranged to be selectively engaged or disengaged in shift-up operation. In the control apparatus, a shift timing valve is adapted to control fluid under pressure discharged from one of the frictional engagement elements to be disengaged in shift-up operation. The shift timing valve is designed to be switched over when a resultant force of hydraulic pressure applied thereto from the other fictional engagement element to be engaged and pilot pressure applied thereto under control of a solenoid valve becomes larger than that of modulated accumulator back pressure applied thereto in accordance with throttle pressure from a throttle valve and a biasing force of a return spring assembled therein. When the shift timing valve is switched over, it acts to switch over discharge of the fluid under pressure from a slow-drain condition to a quick-drain condition.
In the control apparatus described above, the operation of the solenoid valve is controlled on a basis of a change-speed map wherein a boundary line indicative of an opening degree of an engine throttle in relation to the vehicle speed is determined in a higher speed region than that defined by a shift diagram. In the change-speed map, an overlap region is defined between the boundary line and the shift diagram, and an underlap region is defined in a higher speed region than that defined by the boundary line. When a coordinate defined by the opening degree of the engine throttle and the vehicle speed in shift-up operation is in the overlap region of the map, the solenoid valve is opened to permit release of the pilot pressure from the shift timing valve. When the coordinate is in the underlap region, the solenoid valve is closed to apply the pilot pressure to the shift timing valve.
Under control of the control apparatus, the opening degree of the engine throttle and the vehicle speed are adapted as parameter for selecting the overlap control or the underlap control in shift-up operation of the transmission. This is effective to select a condition for change-speed in accordance with a travelling condition of the vehicle thereby to reduce unpleasant shocks in shift-up operation of the transmission. However, an optimal condition for change-speed may not be obtained since the overlap or underlap control is alternatively selected to select the condition of charge-speed.